NEW YORK – In the end, it was a boost of riboflavin, or vitamin B2, that halted the child's hearing loss and other progressive nerve problems — a treatment designed to overcome the riboflavin transporter defects she had inherited from both parents.

Without it, her symptoms almost certainly would have progressed, and the child would have experienced far worse in time: declining mobility, optic atrophy, and, eventually, inability to breathe without a ventilator, explained Livija Medne, a senior genetic counselor at Children's Hospital of Philadelphia (CHOP).